There are several instances in which detection of a predetermined material is desired. One of those instances is the detection of explosives concealed in the body of a person entering a secured area.
Present day personnel inspection systems designed to locate weapons which are employed at the portals of secured areas, e.g. courthouses, military installations and the like, normally rely upon electromagnetic detection of a mass of metallic material. Such systems have been in use in airports for a number of years. However, the limitations of such systems are becoming increasingly significant. Electromagnetic systems are limited to the detection of metallic items such as conventional handguns and therefore cannot detect the plastic and ceramic weapons now being manufactured and sold. Such electromagnetic systems also cannot form an image of the detected material; they merely respond to a mass of the metal passing the detector. Similarly, such systems are incapable of detecting other contraband, such as drugs or certain chemical explosives.
Many proposed systems have relied upon the ability of millimeter waves (radiation of wavelength between one millimeter and one centimeter, that is, between 30-300 GHz frequency) to penetrate clothing without harm to the wearer. Millimeter waves are generally reflected from metallic objects and can be used to form an image of such objects. The attenuation and reflection characteristics of ceramic and plastic weapons, as well as contraband such as narcotics, are different with respect to millimeter-wave radiation from those of skin, so that it is possible, although it has not previously been practical, to form an image of objects of these materials carried by a person. These characteristics render millimeter waves suitable for detection of ceramic weapons or other contraband concealed beneath the clothing, for example, of an individual seeking to enter a secured area. Millimeter wave technology, however, presents privacy issues and typically does not have the speed necessary for applications where the detection system is placed at the portal to a secured area.
Other explosives-detection technologies that are based on x-ray techniques measure the x-ray attenuation of the materials that make up the baggage. A number of techniques utilizing neutrons have been proposed for explosives-detection. However, both x-ray based technologies and a neutron based technologies pose safety hazards when employed to detect concealed materials carried by a human or animal.
Therefore and there is need for systems to detect concealed materials which can be both safe to use on humans and fast enough for applications at portals to secured areas.
It is a further need to provide explosive detection techniques that present fewer privacy issues.